You can't set an API level prior than the minimum available, but you can (and should) set one of the available ones.
This page has a pretty exhaustive explanation, including the most common case where you don't specify an API version (either in the calls or in the SDK that you are using):
What happens if I don't specify a version for an API?
We refer to this as an unversioned call. An unversioned call will
default to the oldest available version of the API. Consider this
hypothetical life cycle of API versions:

An unversioned call will always point to the oldest version still
available at the top of the chart. This is currently v2.1, but after
two years it'll be v2.2, then v2.3, etc.
Because of this, our recommendation is to always specify versions when
making calls, where possible.
When using the JavaScript SDK for Facebook you can't make unversioned
API calls.